Spray dryer absorption process for flue gas with entrained coarse particles

ABSTRACT

Disclosed herein is a method for reducing the amount of coarser particles from a stream of flue gas prior to treatment by a spray dryer absorption process (SDA). The method comprises the steps of directing a stream of flue gas containing entrained particles of varying sizes through a curve, thereby subjecting the particles to a centrifugal effect, collecting the particles predominately of the coarser size at a particles collecting means provided in the outer circumference of the curve, dispersing the flue gas reduced in particles of coarse particle sizes into the chamber of a spray dryer absorption apparatus, wherein the particles content of the original flue gas stream is above 20 g/Nm 3 . Also an apparatus for performing the method and a gas disperser is disclosed. The method substantially reduces the cost of the entire plant for treating flue gas having a high content of entrained coarse particles by eliminating the need for a particle pre-collector upstream the SDA.

INTRODUCTION

The present invention relates to a method for reducing the unwantedeffect of coarser particles from a stream of flue gas being treated by aspray dryer absorption process, a spray dryer absorption apparatus fortreating a stream of flue gas containing entrained particles, and a gasdisperser unit for reducing the amount of coarser particles from astream of flue gas prior to treatment by a spray dryer absorptionprocess.

BACKGROUND FOR THE INVENTION

Combustion plants emit large amount of flue gas containing fly ash andharmful gasses, such as SO₂. Therefore, to avoid undesired impact on theenvironment, combustion plants usually have installed flue gasdesulfurization systems to reduce SO₂ emission and units reducing theparticulate emission.

A particular attractive way of removing particles as well as acidiccomponents from a flue gas is provided by a spray dryer absorber (SDA).In an SDA acidic gasses are absorbed in an absorbent sprayed into theflue gas. Typically, the flue gas is introduced into a spray dryerabsorption chamber and contacted with a fine spray of absorbent slurry,such as an aqueous suspension of burned lime. A significant part of theacidic components in the flue gas are rapidly absorbed into the alkalinedroplets and water is evaporated simultaneously. The gas distribution,slurry flow rate and droplet size are controlled such that the dropletsare dried to a fine powder.

U.S. Pat. No. 4,279,873 (Niro) discloses an SDA of the type disclosedabove, which further have been improved by partly recycling of the finepowder produced in the chamber of the SDA. The recycling results in theuse of less absorbent because unreacted alkaline absorbent, such asCa(OH)₂, remains in the fine powder.

Poor quality coal and other fossil combustibles generate significantamounts of ash as a by-product of combustion. This ash typically iscollected at the bottom of the furnace but significant quantities as flyash can become entrained in the flue gas stream exiting the furnace.

The fly ash is highly abrasive and can abrade ducting, guide vanes,dampers and other components of the SDA. The ash particles collide withthe apparatus components and causes extensive surface abrasion. Inadvanced stages of abrasion the components get perforated and may failonce they lose their structural integrity. Such abrasion shortens theservice-life of the SDA components. When damaged components have to bereplaced, the plant, e.g. a power station, has to be shut down. Theresulting costs are not only the costs of replacing the components butalso the cost of stoppage of power production.

To reduce the wear of sensitive parts of the gas disperser apre-collector to filter out fly ash may be required. U.S. Pat. No.5,738,711 suggests using a hopper with a deflector and a screeningarrangement to separate fly ash from the flue gas exiting a furnace.Others have suggested the use of electrostatic precipitators, cyclonesor bag filters to reduce the content of fly ash in the flue gas.

U.S. Pat. No. 4,530,822 shows some embodiments of an SDA and the use ofdifferent atomizing means such as a rotary atomizer wheel or nozzles.

The present invention is based on the assumption that small amounts offly ash, such as below 20 g/Nm³, are acceptable. However, beyond thatthe fly ash creates too much wear on the gas disperser which feeds theflue gas into the SDA. Hitherto, the excessive amounts of fly ash havebeen separated out in external pre-collectors, which, however,complicate the entire process and contribute to the total costs of theplant. It is the aim of the present invention to substantially reducethe cost of the entire SDA plant for treating flue gas having a highcontent of entrained particles. Lower cost of the SDA system may promptareas rich in coal of a poor quality to control the air pollution byusing a SDA system for treating the flue gas. A further aim is to reducethe “foot print” of the plant, i.e. the ground area needed for erectingthe plant.

SUMMARY OF THE INVENTION

The present invention relates to a method for reducing the amount ofcoarser particles from a stream of flue gas prior to treatment by aspray dryer absorption process, comprising the steps of:

directing a stream of flue gas containing entrained particles of varyingsizes through a curve, thereby subjecting the particles to a centrifugaleffect,

collecting the particles predominately of the coarser size at aparticles collecting means provided in the outer circumference of thecurve,

dispersing the flue gas reduced in particles of coarse particle sizesinto the chamber of a spray dryer absorption apparatus,

wherein the particles content of the original flue gas stream is above20 g/Nm³.

It has been realized by the present inventors that not only the amountof particles is important for the abrasion, also the size of theparticles is of significance for the abrading properties of the fluegas. Thus, a purpose of the invention is to reduce the amount of coarserparticles in the flue gas before the flue gas comes into contact withsensible parts of the SDA. The particles of the present inventioncomprise dust particles or fly ash formed directly by the combustion offossil fuel as well as particles present in the flue gas from otherorigins, such as solid carrier materials or additives from fluid bedcombustion.

The curve through which the stream of flue gas is directed is curved atleast 90 degrees. In a preferred embodiment, the curve is at least 180degrees, such as at least 270 degrees. The curve may be designed suchthat the conduit is narrowed along the direction of the flue gas. In apreferred embodiment the curve is a scroll in which the cross-sectionalarea of the duct diminishes constantly along the path of the flue gasstream. At the inner circumference of the scroll guide vanes arepositioned to disperse the flue gas in a desired flow pattern around anatomizer. The narrowing of the scroll is suitably designed to obtain asubstantially constant flow rate of the flue gas inside the curve.

Usually, the inner diameter of the scroll is at least 2 meter but may be15 m or more. The outer radius at the inlet may be 2 m or more but isusually not above 13 m. The flue gas introduced into the scroll may havea velocity dependent on the design of the scroll. A typical velocity maybe in the range of 15 to 18 m/s.

The scroll is usually supplied with flue gas by a single inlet. However,it may in certain embodiments be an advantage to introduce flue gas intothe scroll at several positions. Also, one SDA may have more gasdispersers in the top part, e.g. three.

The particles having been subjected to a centrifugal force are collectedat the outer circumference of the curve in one or more particlecollection means, which herein also may be referred to as ash traps ordust traps. When a single particles collecting means is used the lengthof the curve is suitably 180 degrees or more. The particles collectingmeans may be positioned after a curve of 45 to 135 degrees to collectthe largest particles. If, however, the particles collecting means ispositioned between 135 and 225 degrees a higher amount of the largestparticles may be catched. In addition a substantial amount of themid-sized particles is collected too. If a single particles collectingmeans is used, it is preferably positioned between 225 and 315 degreesto catch a high amount of the coarser particles abrading the componentsof the device.

In an aspect of the invention, two particles collecting means areprovided in the curve. The first particles collecting means may beprovided to collect the largest particles after a curve of 45 to 135degrees and the second particles collecting means may be provided after225 to 315 degrees to collect a part of the remaining particles. In theevent a third particles collecting means is provided, it is usuallysituated between the first and the second, i.e. between 135 and 225degrees. The use of two or more particles collecting means has theadvantage that the wear of the scroll itself is reduced and the wear onthe particles collecting means itself is distributed.

The optimum number of installed particle collection means in a gasdisperser is dependent on a number of parameters, of which the mostimportant is the particle loading in the inlet flue gas. Otherparameters, which may influence the selection of number of particlecollection means are: flue gas amount (size of gas disperser), gasdisperser type, particle size distribution and abrasiveness ofparticles.

Experiments have shown that 2 particle collection devices often are theoptimum number. The first Particle Collection Device is often located onthe outer perimeter of the scroll at around 90° from the inlet.

The first particle collection means has the primary purpose ofcollecting the very coarse particles, mainly in order to avoid abrasionon the scroll itself.

The second particle collection means is often located on the outerperimeter of the scroll at around 270° from the inlet. The secondparticle collection device has the main purpose of collecting theremaining very coarse particles and the medium size particles, mainly toavoid abrasion on the various sets of guide vanes leading the flue gasfrom the scroll into the absorber vessel.

Easier jobs may be handled with only 1 particle collection means,whereas very difficult jobs may require as many as 4 or 6 particlecollection means.

The particles entering the flue gas disperser are of varying sizes.Often, the particle sizes entering the curve is distributed such that 10vol % of the particles are of a size above 100 μm. The invention issuitable for treating flue gasses with a substantial amount of abrasiveparticles having a size of above 100 μm.

The flue gas may originate from any combustion process, includingcombustion of fossil fuel, incineration of waste, or from otherprocesses generating industrial off-gases. The method of the inventionis especially suitable to treat flue gasses from combustion of coalsthat result in a high amount of fly ash.

The method of the invention may reduce the amount of the coarserparticles from a stream of flue gas to a significantly lower level. In apreferred aspect 50% or more of the particles having a size of 150 μmand above is collected at the particles collecting means. In a morepreferred aspect the amount of collected particles is 60% or more of theparticles having a size of 150 μm and above.

In a further aspect of the invention it relates to a spray dryerabsorption apparatus for treating a stream of flue gas containingentrained coarse particles, comprising a gas disperser for entering theflue gas into a spray dryer absorber chamber containing an atomizer forspraying an absorbent slurry, and an outlet for the treated flue gas,

wherein the gas disperser comprises a curved conduit through which thestream of flue gas is guided, thereby affecting the entrained particleswith a centrifugal effect, a particles collecting means positioned atthe outer perimeter of the curved conduit for collecting the particles,and particles positioning means for assisting in the conveying theparticles to the particles collecting means.

The SDA apparatus of the present invention has the advantage that apre-collector may be omitted even when the content of particles in theflue gas stream is high, i.e. above 20 g/Nm³. Depending on the particleorigin such content may be above 25 g/Nm3, above 30 g/Nm3, above 40g/Nm3 or above 50 g/Nm3. Despite the absence of a pre-collector forparticles, the risk of abrasion is substantially reduced due to thereduced amount of coarser particles.

The particles collecting means positioned at the outer perimeter of thecurve may have various designs. In a first embodiment, the particlescollecting means comprises a slot in the outer perimeter and a duct forconveying the collected particles. The slot may extend in the entireheight of the curved conduit or only a part thereof. Since the particlesof the larger size are supposed to be conveyed at the lower part of thecurved conduit and it is desired not to withdraw more flue gas thannecessary, it is preferred to let the slot extend only partly from thebottom of the conduit towards the ceiling. Thus, it is preferred to letthe slot extend from the bottom of the conduit and half the way or lessto the ceiling. In a certain aspect of the invention the slot extends30% or less from the bottom towards the ceiling. Usually, the slotextends at least 5% from the bottom towards the ceiling. The slot isusually connected to a duct allowing the particles exiting through theslot tangentially to move into the duct.

The exact sizing of the slot type particle collection means is definedby the width (W) of the slot and by a height/width (H/W)-ratio of theslot. The H/W ratio is optimally selected depending on variousparameters, of which the most important are the particle sizedistribution and the flue gas velocity. A typical H/W-ratio is about 2,but some designs may involve H/W-ratio as low as 1 or H/W ratio of 3 oreven higher. An option is to extend the slot all the way to the roof ofgas disperser scroll and thereby possibly eliminate the need for wallguide tracks. The H/W-ratio could in this scenario be higher than 10.

The width may be dependent of the gas disperser size. The width, theH/W-ratio and the number of particle collection means define the totalflue gas outlet area via the particle collection means. This area willbe selected to give the desired particle collection without bypassing anexcessive amount of flue gas around the gas disperser inner part. Theamount of flue gas passing through the particle collection devicesshould normally be in the range 0.5-10.0% of the total amount of fluegas.

Some typical dimensions for a Particle Collection Device at a typicalgas disperser treating between 100.000 to 500.000 kg flue gas per hourwill be about H=200 mm and about W=100 mm. Smaller dimension areselected at smaller gas dispersers, larger dimensions are selected forlarger gas dispersers. The parameters influencing the selection of thedimensions for the particle collection devices may beside the gasdisperser size be: number of particle collection devices, particleloading and particle size distribution.

In another design of the particles collecting means, it comprises adeflector in the outer perimeter and a duct for conveying the collectedparticles. The deflector usually extends from the inner face of theouter perimeter towards the centre of the curve and has the purpose ofdisturbing the path of the particles entrained by the stream of fluegas. The deflector may extend entirely from the bottom of the conduit tothe ceiling or only a fraction thereof. In a preferred aspect it isdesired to have the deflector only to extend from the bottom and upwards70% or less towards the ceiling of the curved conduit.

When the particles collides with the deflector it is presently believedthat a substantial amount of the particles lose energy and drops to thebottom of the conduit in front of the deflector. The bottom of thecurved conduit is provided with an opening allowing for the extractionof the particles. The opening is connected to a duct for transportationof the particles.

The embodiment using a slot may be preferred over the deflectorembodiment because some particles are suspected to bounce off thedeflector plate and re-enter the scroll flow. In addition some particlesmay follow the air flow around the particles collecting meansconstruction and thus avoid capture by the trap. The slot embodimentavoids the bouncing and channels the particles out of the gas disperserbefore their direction is changed. However, the reflector embodiment maybe less costly.

The duct from the particles collecting means may convey the collectedparticles to the top or the bottom of the SDA chamber. Alternatively,the fly ash is led out of the SDA apparatus e.g. to a cyclone. Usually,it is not desired to convey the collected particles out of the SDAchamber.

Since the authorities may demand for continued running of the SDA allyear to satisfy the emission rules according to which even short pausesin the flue gas cleaning cannot be tolerated, the slot type particlescollecting means are suitably accessible for service from the outside ofthe spray dryer absorption apparatus.

The flue gas enters the gas disperser through an inlet having a fairlylarge cross-section. It is assumed that the particles are affecteddifferently by the centrifugal force across the cross-section.Therefore, in some embodiments of the invention, it may be expedient toprovide particles positioning means in the gas disperser main inlet orupstream hereof to guide the particles in a certain direction.

A particles positioning means in the inlet area may for instance be adiverter plate positioned at the gas disperser main inlet for directingthe inlet flue gas in the direction of the outer perimeter. When thecurved conduit is part of a scroll, the diverter plate is suitablyplaced so as to allow a relatively small amount of the flue gas that hascirculated once to re-enter the scroll together with the fresh stream offlue gas. By appropriately adjustment of the diverter plate it has beenpossible to reduce or eliminate excess wear observed in the vicinity ofthe re-entering area.

According to another embodiment, the particles positioning means is anadditional curved conduit positioned immediately upstream on the inletof the scroll. The additional curved conduit may be regarded as anextension of the scroll or a for-rotation of the flue gas. The extendedscroll consist of a piece of ductwork located upstream the inlet flangeto the gas disperser scroll. This piece of ductwork turns the flue gasin the same direction as the scroll. The turning angle is normallybetween 30° and 120°. The preferred turning angle of the extended scrollis around 90°. When an extended scroll is used, the particles collectingmeans may be positioned differently to collect a large part of the mostheavy particles shortly after or at the entrance of the scroll.

The particles position means may also be present in the interior of thecurved conduit, such as the scroll. In a suitable embodiment, theparticles positioning means is a floor guide track, in which a rod,preferably abrasive resistant, is attached to the bottom of the curvedconduit to assist the movement of the particles towards the outerperimeter of the curved conduit. The rod may be steel profiles orsimilar means extending from the floor of the curved conduit. The rodmay have any suitable cross-sectional shape such as a rectangle or anL-form. A square form of the rod has shown to be useful. The mainpurpose of the rod is to guide the particles that no longer areentrained in the flue gas but moves at a lower velocity at the bottompart of the curved conduit. The floor guide track ends short before itreaches the outer perimeter of the curved conduit. Usually a shortdistance between the rod and the wall in the order of 5-20 cm isprovided to avoid wear due to bombardment of the outer wall withparticles.

The rod is preferably at least partly hollow. If the material of the rodis abraded to form an opening, the particles will move into this openingand thereby prevent that the other side of the rod material is abraded.In this way the particles function as an abrasion layer.

The rod may be straight or curved. In a preferred aspect the rod isconvexly curved in the flow direction of the stream of flue gas. It isgenerally suitable to have the rod to extend in almost the entire lengthfrom the inner gas disperser to the outer wall of the curved conduit.The spray dryer absorption apparatus may be provided with a single, 2,3, 4, or more rods to obtain the desired effect.

In a preferred embodiment the particles positioning means is of an atleast partly annular shape surrounding a part or all of the gasdisperser guide vanes. The annular elevation prevents some of theparticles from entering the sensible vanes of the gas disperser. Thestraight or curved rod may be positioned with one end close to theannular extension to transport the particles away from the vicinity ofthe gas disperser toward the outer perimeter of the curved conduit.

According to a further embodiment the particles positioning means is aplurality of rods, wall guide tracks, attached to the outer perimeter ofthe curved conduit, said rods being essentially perpendicular to thestream of flue gas. The purpose of the essentially vertical abrasiveresistant rods is to guide the particles to the bottom of the curvedconduit.

The various measures for the particles positioning means may be combinedto obtain the desired effect. Experiments have shown that a combinationof a diverter plate and floor guide tracks has resulted in a markedlyreduced wearing of the gas disperser.

The invention also relates to a gas disperser unit for reducing theamount of coarser particles from a stream of flue gas prior to treatmentby a spray dryer absorption process. The gas disperser unit comprises acurved conduit through which the stream of flue gas is guided, therebyaffecting the entrained particles with a centrifugal effect, a particlescollecting means positioned at the outer perimeter of the curved conduitfor collecting the particles, and particles positioning means forassisting in the conveying the particles to the particles collectingmeans.

BRIEF DISCLOSURE OF THE FIGURES

FIG. 1 discloses a detail of a scroll viewed from outside below.

FIG. 2 shows in a perspective view a slot in a part of the inner face ofthe outer perimeter of a scroll.

FIG. 3 depicts the gas disperser unit provided with two slot particlescollecting means viewed from below.

FIG. 4 shows schematically a deflector viewed from above.

FIG. 5 shows a deflector mounted on the inner face of the outerperimeter of a scroll.

FIG. 6 shows a deflector extending half way up to the ceiling.

FIG. 7 shows the gas disperser unit provided with two deflectorparticles collecting means viewed from outside below.

FIG. 8 shows a scroll with a diverter plate in perspective from above.

FIG. 9 shows a scroll with an extension in perspective from above.

FIG. 10 shows a scroll having curved floor guide tracks.

DETAILED DISCLOSURE OF THE INVENTION

The avoidance of harmful emissions from power plants and wasteincinerators has high priority in the political debate. The presentinvention offers an option for lowering the costs for plants producinghigh a mounts of particles from combustion of fossil fuels, includingcoal, but may also be used relating to incineration of waste, or fromother processes generating industrial off-gases

Coal exists in various forms and any of these can be used for providingthe flue gas used in the method of the present invention. The term coal,as used herein, covers peat, lignite (also referred to as brown coal),sub-bituminous coal, bituminous coal, anthracite, and graphite.

FIG. 1 shows a part of a curved conduit 1 from the outside provided witha particles collecting means of the slot type. The slot 2 is shown inFIG. 2 in the lower part of the vertical wall 3 defining the outerperimeter of the conduit. The slot is connected to a duct 4 fortransportation of the flue gas with entrained particles. The duct has arectangular cross section and the inner opening of the duct correspondsto the dimension of the slot. The duct is provided tangential to thecurved conduit to allow the particles to exit the gas disperser unitwithout being deflected. The duct 4 may be provided with a replaceablewear resistant insert.

FIG. 3 shows the particles collecting means of the slot type provide ona flue gas disperser for an SDA. The gas disperser is viewed inperspective from below and is suitable for top mounting in a SDA system.The flue gas preferably enters directly into the gas disperser unit ofthe invention without being pre-treated to remove a part of theparticles. The entrance 5 of the flue gas occurs through a conduithaving a rectangular cross-section. The capturing effect may be furtherimproved by using particle positioning means in the inlet 5 or upstreamhereof. The curved form of the conduit subjects the entrained particlesto a centrifugal effect. Since the centrifugal force depends on the sizeof the particles it is assumed the particles with a large size will havea tendency to be deflected to a minor degree, whereas minor particles toa greater extend will be entrained by the streaming flue gas.

Two particles collecting means have been provided in the perimeter ofthe conduit. The first particles collecting means is provided after theconduit has curved around 90 degrees and the second particles collectingmeans is provided after the conduit has curved about 270 degrees. Thehorizontally ducts 4 meet vertically ducts 6, which convey the flue gaswith entrained particles to the SDA chamber. These ducts may havereplaceable wear resistant lining. Such replacement may be performedfrom the outside and during operation. The ash is self-transporting dueto the flue gas flow in the particles collecting device. The main partof the flue gas depleted of coarse particles is led through guide vanesand enters the SDA chamber at outlet 7. In the centre, an atomizer, arotary atomizer wheel or nozzles, (not shown) is provided for sprayingdroplets of the absorbent into the dispersed flue gas. The form of thecurved conduit resembles a snail shell and is generally referred toherein as a scroll.

FIG. 4 shows a top view of a deflector mounted in the inner face of thecurved conduit. The deflector comprises an oblong vertical plate 8extending from the bottom of the curved conduit towards the ceiling. Theoblong vertical plate is positioned in the conduit essentiallyperpendicular to the direction of the flue gas flow. The particlescollecting means further comprises a shielding plate 9 to preventparticles that has collided with the vertical plate 8 to re-enter theflue gas stream. The shielding plate is mounted perpendicular to theplate 8 and in parallel with the flow direction. A plate 10 may beplaced to prevent unwanted turbulence. A circular opening 11 allow theparticles captured by the trap to exit the scroll. Usually, the innerface of the particles collecting means is covered with a wear resistantsurface, e.g. a coating, a special metal or ceramic, a bushing, ceramictiles or the like. The presence of the tiles makes it easy to servicethe particles collecting means since the used tiles may simply bereplaced with new tiles. The opening in the floor of the curved conduitis connected to duct 12 for conveying the particles to the SDA chamber.

The deflector type particles collecting means is shown in twoembodiments in FIG. 5 and FIG. 6 in a perspective view. In FIG. 5, thetrap formed by the vertical plate 8 and the shielding plate 9 extendsfrom the bottom to the ceiling of the curved conduit. The embodimentshown in FIG. 6 extends only half the way towards the ceiling.

FIG. 7 shows the gas disperser in a view from below. Two particlescollecting means have been provide in the perimeter of the conduit. Thefirst particles collecting means is provided after the conduit hascurved around 90 degrees and the second particles collecting means isprovided after the conduit has curved about 270 degrees. The verticalducts 12 convey the flue gas with entrained particles to the SDAchamber. The main part of the flue gas depleted in coarser sizedparticles is forced through guide vanes and enters the SDA chamber atoutlet 7.

It should be understood that any type of trap whether using a slot or adeflector or another design may be combined with ducts as shown in FIG.3 and FIG. 7, or other ducting designs.

FIG. 8 shows a partly transparent perspective view of the scroll. In theinlet area a diverter plate 13 is positioned for directing the inletflue gas in the direction of the outer perimeter. The diverter plate isplaced so as to shield the guide vanes from the particles in the fluegas. Typically, the diverter plate is a square having a length of theedges of 0.5 to 1.5 m depending on the size of the equipment. Byappropriately adjustment of the diverter plate excess wear observed inthe vicinity of the re-entering area is avoided.

The diverter plate may be regarded as an extension of the scroll wall.The presence of the diverter plate reduces the tendency of particleimpingement on the guide vanes where the two streams of gases are mixed.The particle impingement otherwise may result in excessive abrasion inthe area in the vicinity of the mixing of the gas streams.

In FIG. 8 a straight rod 14 as a floor guide track is welded to thebottom of the scroll. The straight rod extents in the lower part of thescroll from the lower part of the diverter plate to or close to thevertical outer wall of the scroll. The straight rod assists the movementof the particles towards the outer perimeter of the scroll. The straightrod is prepared of a hollow steel profile. The steel profile has e.g. a0.1×0.1 m square form.

The bottom of the scroll is provided with 3 further floor guide tracks.These tracks extend from close to the guide vanes of the gas disperserto close to the inner face of the outer wall of the scroll. The floorguide tracks may be curved or straight with rather similar results butcalculations have shown that the curved guide tracks are superior.

FIG. 9 shows a scroll with an additional curved conduct 15 arrangedimmediately upstream of the inlet flange. The extended scroll has afurther 90° turn providing for a prolonged centrifugal effect of theparticles entrained in the flue gas stream.

FIG. 10 shows a view from above of a gas disperser scroll. The floor ofthe gas disperser scroll is provided with evenly distributed curvedguide tracks 16. The guide tracks are prepared of a hollow profile steelrod that has been bent. The curved guide tracks are convex in the flowdirection of the flue gas to obtain an efficient transportation of theparticles along the rod and an efficient delivery at the inner face ofthe outer scroll wall.

The annular guide vanes 18 of the disperser positioned centrally in thescroll are surrounded by an annular elevation 17. The annular elevationprevents some or all of the particles from entering the sensible vanesof the gas disperser. The curved rods 16 are positioned with one endclose to or abutting the annular elevation to guide particles at theedge of the elevation towards the inner face of the outer wall of thescroll. The annular elevation has a height similar to the height of thecurved rods.

EXAMPLES Example 1

The particle size distribution of the fly ash received from a powerplant or a waste incinerator may differ considerably from case to case.An example of such distribution from a power plant is given below.

Less than Cumulated (μm) frequency (%) 3 8 7 18 13 33 20 46 31 59 40 6452 67 150 91 300 100

The coarse fraction of the particles is generally more abrasive than thefine fraction, so the bigger the mean particle size or the coarsefraction is, the bigger the wear problem is.

In a typical 300 MW power plant the flue gas amount entering the SDAwith a typical diameter of 17 m may be about 1·10⁶ Nm3/h. Assuming theflue gas contains 30 g/Nm³ fly ash, the total amount of fly ash is 30ton/h.

Computer simulations have been performed based on three embodiments:slot type particles collecting means (FIG. 1, FIG. 2, and FIG. 3),deflection type particles collecting means (FIG. 4, FIG. 5, and FIG. 7)with full deflector height, deflection type particles collecting means(FIG. 4, FIG. 6, and FIG. 7) with half deflector height. Thecalculations were performed on particles having the diameters 10, 50,100, 150, and 200 μm, approximate spherical form, and a density of 2.6g/cm³. The flue gas inlet flow velocity was set to 16.6 m/s and the fluegas was estimated to a density of 0.882 kg/m3 and a viscosity of2.314·10⁻⁵ Pa s.

The result of computer simulations is shown in tables 1 to 4.

TABLE 1 Deflector type particles collecting means-full height d 1^(st)ash trap 2^(nd) ash trap Total recovered μm % recovered particles 10 0.20.2 0.5 50 3 3 6 100 13 0.2 13 150 28 0 28 200 40 23 63

TABLE 2 Deflector type particles collecting means-half height d 1^(st)ash trap 2^(nd) ash trap Total recovered μm % recovered particles 10 0.20.3 0.5 50 2 5 8 100 10 27 37 150 21 37 58 200 3 32 65

TABLE 3 The slot type particles collecting means d 1^(st) ash trap2^(nd) ash trap Total recovered μm % recovered particles 10 1 1 3 50 410 14 100 13 43 55 150 25 43 68 200 37 38 75

TABLE 4 Slot type particles collecting means using diverter plate andcurved floor guide tracks. d 1^(st) ash trap 2^(nd) ash trap Totalrecovered μm % recovered particles 10 1 1 3 50 4 11 15 100 17 48 65 15032 62 94 200 43 56 99

The computer simulations indicate that each of the three embodiments isable to reduce the amount of coarser particles in the flue gas. However,the amount of particles removed by the particles collecting means isbest for the slot type particles collecting means. It appears that thesuperiority applies for all particle sizes. It is believed that thereason is that the slot type particles collecting means channels theparticles out of the gas disperser before they are directed downwards.The deflector type particles collecting means appears to be lessfavourable because colliding particles tend to bounce away into the gasstream instead of being directed to the outlet. The slot type particlescollecting means may be drastically improved for particles above 100 μmby using a diverter plate and curved floor guide tracks in combination

By combining the slot type particles collecting means with positioningmeans in the gas disperser, the recovery efficiency increasessignificantly. Using only floor guide tracks or a diverter plate aspositioning means will increase the recovery efficiency, but a higherefficiency is obtained with a combination of means. The extended scrollmay be used with or without further positioning means.

In addition to the floor guide tracks, wall guide tracks may beinstalled for guide of particles towards the floor. The wall guidetracks are most beneficial in smaller gas dispersers, but may beinstalled in all dispersers irrespective of the size.

1. A method for reducing the amount of coarser particles from a streamof flue gas prior to treatment by a spray dryer absorption process,comprising the steps of: directing a stream of flue gas containingentrained particles of varying sizes through a curved conduit of a gasdisperser, thereby subjecting the particles to a centrifugal effect,conveying the particles to particles collecting means using particlespositioning means, collecting the particles predominately of the coarsersize at the particles collecting means provided in the outercircumference of the curve, dispersing the flue gas reduced in particlesof coarse particle sizes from the gas disperser into the chamber of aspray dryer absorption apparatus, said chamber containing an atomizerfor spraying an absorbent slurry and an outlet for the treated flue gas,wherein the particles content of the original flue gas stream is above20 g/Nm³.
 2. The method according to claim 1, wherein the flue gasstream is directed through a curve of at least 90 degrees.
 3. The methodaccording to claim 1, wherein particles are collected at two or moreparticles collecting means positioned along the curve.
 4. The methodaccording to claim 1, wherein the particle sizes entering the curve isdistributed such that 10 vol % of the particles are of a size above 100μm.
 5. The method according to claim 1, wherein the flue gas streamoriginates from a plant combusting fossil fuel.
 6. The method accordingto claim 1, wherein 50% or more of particles having a size of 150 μm andabove is collected.
 7. A spray dryer absorption apparatus for treating astream of flue gas containing entrained coarse particles, comprising agas disperser for entering the flue gas into a spray dryer absorberchamber containing an atomizer for spraying an absorbent slurry, and anoutlet for the treated flue gas, wherein the gas disperser comprises acurved conduit through which the stream of flue gas is guided, therebyaffecting the entrained particles with a centrifugal effect, a particlescollecting means positioned at the outer perimeter of the curved conduitfor collecting the particles, and one or more particles positioningmeans for assisting in conveying the particles to the particlescollecting means.
 8. The spray dryer absorption apparatus according toclaim 7, wherein the curved conduit is curved at least 90 degrees. 9.The spray dryer absorption apparatus according to claim 7, wherein thecurved conduit is part of a scroll having a cross-sectional areadiminishing along the path of the flue gas stream.
 10. The spray dryerabsorption apparatus according to claim 7, wherein two or more particlescollecting means are positioned along the perimeter of the curvedconduit.
 11. The spray dryer absorption apparatus according to claim 7,wherein the particles collecting means comprises a slot in the outerperimeter and a duct for conveying the collected particles.
 12. Thespray dryer absorption apparatus according to claim 7, wherein theparticles collecting means comprises a deflector in the outer perimeterand a duct for conveying the collected particles.
 13. The spray dryerabsorption apparatus according to claim 7, wherein the collectedparticles are conveyed to the top of the spray dryer absorber chamber.14. The spray dryer absorption apparatus according to claim 7, whereinthe collected particles are conveyed to the bottom of the spray dryerabsorption chamber.
 15. The spray dryer absorption apparatus accordingto claim 7, wherein the collected particles are conveyed out of thespray dryer absorption apparatus.
 16. The spray dryer absorptionapparatus according to claim 7, wherein particles positioning means isprovided in the gas disperser main inlet or upstream hereof.
 17. Thespray dryer absorption apparatus according to claim 7, wherein theparticles positioning means is a diverter plate positioned at the gasdisperser main inlet for directing the inlet flue gas in the directionof the outer perimeter.
 18. The spray dryer absorption apparatusaccording to claim 7, wherein the particles collecting means areaccessible for service from the outside of the spray dryer absorptionapparatus.
 19. The spray dryer absorption apparatus according to claim7, wherein the particles positioning means is an additional curvedconduit positioned immediately upstream on the inlet of the scroll. 20.The spray dryer absorption apparatus according to claim 7, wherein theparticles positioning means is a floor guide track, in which a rod isattached to the bottom of the curved conduit to assist the movement ofthe particles towards the outer perimeter of the curved conduit.
 21. Thespray dryer absorption apparatus according to claim 7, wherein the rodis convexly curved in the flow direction of the stream of flue gas. 22.The spray dryer absorption apparatus according to claim 7, wherein therod is at least partly hollow.
 23. The spray dryer absorption apparatusaccording to claim 7, wherein the particles positioning means is of anannular shape surrounding the gas disperser.
 24. The spray dryerabsorption apparatus according to claim 7, wherein the particlespositioning means is a wall guide track in which a rod is attached tothe outer perimeter of the curved conduit, said rod being essentiallyperpendicular to the stream of flue gas.
 25. The spray dryer absorptionapparatus according to claim 7, wherein two or more particlespositioning means are selected.
 26. A gas disperser unit for reducingthe amount of coarser particles from a stream of flue gas prior totreatment by a spray dryer absorption process, comprising a curvedconduit through which the stream of flue gas is guided, therebyaffecting the entrained particles with a centrifugal effect, a particlescollecting means positioned at the outer perimeter of the curved conduitfor collecting the particles, and particles positioning means forassisting in the conveying the particles to the particles collectingmeans.
 27. The disperser according to claim 26, wherein the curvedconduit is curved at least 90 degrees.
 28. The gas disperser accordingto claim 26, wherein two or more particles collecting means arepositioned along the perimeter of the curved conduit.
 29. The gasdisperser according to claim 26, wherein the particles collecting meanscomprises a slot in the outer perimeter and a duct for conveying thecollected particles.
 30. The gas disperser according to claim 26,wherein the particles collecting means comprises a deflector in theouter perimeter and a duct for conveying the collected particles. 31.The gas disperser according to claim 26, wherein the particlepositioning means is provided in the gas disperser main inlet orupstream hereof.
 32. The gas disperser according to claim 26, whereinthe particles positioning means is selected among one or more of thegroup consisting of (a) a diverter plate positioned at the gas dispersermain inlet, (b) an additional curved conduit positioned immediatelyupstream on the inlet, (c) one or more floor guide tracks, (d) anannular shaped rod surrounding the gas disperser, and (e) one or morewall guide tracks.